The importance of both intrinsic properties of neurons and their synaptic interactions in shaping network output has been extensively shown through previous work. Intrinsic properties not only shape activity and response properties of neurons, but ion channel expression can vary 2-5 fold across preparations in the same identifiable cell, while the network continues to produce a stereotyped behavioral output. It has also been suggested that synaptic strength varies 2-5 fold, based on computational studies. My central hypothesis is that the synapses in the leech heartbeat CPG will vary in strength over a 2-5 fold range. Despite this variability in strength, the relative strength of connections onto the same postsynaptic target will be maintained. An important balance must exist between inhibitory chemical synapses and excitatory electrical coupling to reliably generate stereotyped phasing. In each of my aims synaptic strength will be measured with postsynaptic single electrode voltage clamp at a standard holding potential. Specific Aim 1: To measure the strength of all synapses within the timing circuit of the leech heartbeat CPG and their output onto the switch interneurons. Specific Aim 2: To measure the strength of the synapses from the timing circuit and the switch interneurons onto the middle premotor interneurons. Determine how a balance is created between excitatory electrical and inhibitory chemical connections. .Specific Aim 3: To determine how the newly characterized rear heart interneurons in ganglia fifteen and sixteen connect to the rest of the circuit by measuring their intra- and inter-ganglionic electrical coupling and the strength of their connection to the other CPG interneurons. Computational studies have shown that increasing variability in a neural network will substantially decrease the synchrony of the network. This decreased synchrony could be a preventative mechanism for epilepsy in networks with high levels of variability. This is also suggestive that there may be other neurological disorders related to the levels of variability within the nervous system, and therefore it will be important to explore this variability starting in a simple nervous system. PUBLIC HEALTH RELEVANCE: A basic understanding of how sets of neurons come together to perform a specific behavior, such as walking or breathing, will be crucial when working with spinal cord injury (SCI) and disease in an effort to restore critical behaviors. Our ability to construct an accurate computational model of how these networks function will allow us to create prosthetics and implantable devices that may, in the future, restore some of these lost functions.